The use of a dry, particulate toner heat fused to paper to form a permanent image on one or both sides of a copy sheet is well known in xerographic technology. A widely used heat fuser is a hot roller fuser. In this type of fuser, the sheet of paper to be fused passes through the pressure nip formed by two rollers which are in pressure contact. The quality of fusing produced by such a fuser is a function of temperature, time and pressure.
It is desirable to maintain the temperature in the fuser within a range optimal for fusing the toner transferred onto the copy sheet. For this purpose, temperature sensors and controllers have been used. For example, it is known to use a thermal couple directly on or in the vicinity of the heating element in the fuser to maintain the fuser temperature at this optimal point. In addition, U.S. Pat. No. 4,104,692 discloses a device for detecting abnormal temperature in a fuser. U.S. Pat. No. 4,318,612 discloses a control system which compares a command set point temperature to the fuser's actual temperature and energizes a fuser heater accordingly. A cold start of the fuser is distinguished from a warm start and the command set point temperature is magnitude-programmed accordingly. U.S. Pat. No. 4,301,359 discloses temperatures sensing elements for detecting the surface temperature of a heat roller and a control circuit which controls the conduction of the heating element in response to the output from the temperature sensing elements. U.S. Pat. No. 4,078,166 discloses a device for controlling the desired temperature detecting element in a fuser wherein the fuser means is heated to a temperature that allows fusing to be obtained, and then lowered after the copying operation of a few sheets of copy paper is finished. U.S. Pat. No. 3,851,144 discloses system for controlling fuser energization based upon the ambient temperature of the fuser.
There is a certain degree or latitude of the interface temperature between the toner and the paper to obtain suitable tonerpaper fusing. The toner-paper interface temperature is effected by many things such as different varieties of papers, ambient and environment temperature, the type of toner, and the basic fuser design. It is important to compensate for these differences to obtain the correct interface temperature. Adding new paper in the machine that is cooler will necessarily require an increase in the fuser temperature. Paper temperature and ambient environment temperature have important effects on the latitude of roll fusers. It has been determined that if the paper temperature and ambient temperature were well known, the correct toner-paper interface temperature could be more easily controlled. In general, the thermal mass of an internally heated fuser roller precludes response to direct real time sensing of the temperatures of individual sheets. That is, the fusing process speeds are too rapid (with respect to the fuser roll response capabilities) to allow fuser temperature control based on the temperature of each individual incoming copy sheet.
One alternative would be to use multiple sensors to sense both the temperature of the paper in the various paper trays and the temperature of various locations in the machine. This information could then be processed in a microprocessor or controller and used to provide control of fuser roll behavior during both machine run and machine standby. Unfortunately, this is an elaborate and expensive system. In addition to sensor costs, there are increased costs for microprocessor channels, sequencing and wiring. In addition, each sensor introduces some error and the effectiveness of the system is reduced.
A difficulty, therefore, with the prior art fuser temperature control systems has generally been complexity and cost. Furthermore, such systems have not always been accurate and reliable in accounting for the various temperature changes within the machine such as the ambient temperature and the change in temperature of the copy sheet upon which the toner is to be fused. Response to various temperature changes is important in order to continually provide the correct toner paper interface temperature.
It is an object of the present invention, therefore, to provide a new and improved fuser temperature control. It is another object of the present invention to provide a fuser temperature control that provides longer life, energy savings, and in particular maintains fuser offset latitudes, that is the correct toner/paper interface temperature. It is still another object of the present invention to provide a new and improved fuser control that senses both paper temperature and the machine ambient temperature when paper is not present at the sensor to maintain the correct fuser offset latitude.
Further advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.